This invention pertains to improved electrodialysis (ED) apparatus, and pertains more particularly to systems and processes that utilize such apparatus to treat large volumes of liquid. Generally as used herein and in the claims appended hereto, unless the context implies otherwise, the terms electrodialysis or ED shall be understood to be generic, and shall refer to devices for demineralizing, or for removal of components from, a fluid, wherein device units have alternating dilute and concentrate cells defined between sets of ion exchange membranes. These devices treat fluid passing through the dilute cells by transferring components thereof into another fluid present in the concentrate cells. Thus, the term will include various forms of electrodialysis having unfilled inter-membranes spaces or cells—i.e., shall include classical electrodialysis (ED), electrodialysis reversal (EDR); and shall also include filled-cell electrodialysis (now commonly called electrodeionization, EDI, continuous electrodeionization or CDI), and filled cell reversing electrodialysis (EDIR). Of these, unfilled ED gained widespread commercial application at an historically earlier period, and both ED and EDR variants remain principal treatment modalities among electrically-mediated membrane-based ion exchange processes in several large-scale industrial treatment applications such as dairy, fruit juice and plant syrup treatment applications. EDI and EDIR, which rely on intrinsic water splitting to continuously regenerate a packing of exchange resin in the dilute cells, have had more limited application, most prominently in the area of water purification where their susceptibility to fouling or scaling can be controlled by any of several established pretreatment regimens for the feed stock.
Electrodialysis apparatus having unfilled cells defined by a multiplicity of alternating anion selective and cation selective membranes was apparently first historically described by K. Meyer and W. Strauss in 1940 (Helv. Chim. Acta 23 (1940) 795-800). The membranes used in this early ED apparatus were poorly ion selective. The discovery of ion exchange (“IX”) membranes (e.g., as described in U.S. Reissue Pat. RE 24,865), particularly synthetic polymer membranes which had high ion permselectivity, low electrical resistance and excellent stability, led rapidly to the invention of ED systems using such membranes (e.g., as described in U.S. Pat. No. 2,636,852) and to the growth of industries that exploit such apparatus, for example, to desalt brackish water, concentrate sea water, de-ash cheese whey, and to concentrate or otherwise treat modified starch or sugar streams, or more generally, treat aqueous streams and refinery, fermentation or agricultural fluids. During the last forty years approximately five thousand ED plants have been installed on a world-wide basis.
The utility of electrodialysis, both filled and unfilled varieties, continues to be limited to some extent, however, by technical factors, particularly its relatively low limiting current densities and its limited ability to remove poorly ionized substances. These limitations and deficiencies of prior art ED systems are discussed further below.
A. Limiting Current Density:
The IX resins and membranes used in ED are highly selective to ions of one sign or the other. A substantial fraction of the ions passing through the membranes must reach the membrane walls by diffusion from the ambient solution across a thin laminar flow layer that develops along the interfaces between the membranes and the adjacent solution that is being depleted of ions (the “dilute” or “diluting” solution or stream, as it is known in the art). The maximum rate of diffusion of ions through the diluting solution occurs when the concentration of electrolyte at such membrane interfaces is essentially zero. The current density corresponding to such zero concentration at a membrane interface is referred to in the art as the limiting current density. To increase the limiting current density it is necessary to increase the rate of ion diffusion. This may be accomplished by reducing the thickness of the laminar flow layers—for example, by changing some fluid-dynamic parameter—by flowing the ambient solution more rapidly past the membrane surfaces, and/or by using a turbulence promoter, and/or by increasing the temperature of the fluid. The latter approach is especially useful for plant (e.g., botannical) syrups whose viscosity, hence laminar layer behavior, may be highly temperature dependent, but it is further limited by the limited ability of ion exchange resins to withstand higher temperatures without impairment of their exchange functionality. Practical limiting current densities are generally in the range of 5,000 to 10,000 amperes per square meter for each kilogram-equivalent of salts per cubic meter of solution (e.g., 0.5 to 1 amperes per square centimeter for each gram-equivalent of salts per liter). A typical brackish water has a concentration of salts of about 0.05 kg-eq/m3 (e.g., about 0.05-eq/l or about 3000 parts per million (“ppm”)), and therefore has a limiting current density in the range of about 250 to 500 amperes per m2 (0.025 to 0.05 amperes per cm2). In many food industry applications, the salt concentration may be modified to suit a processing requirement, but often the added salt must be removed in a subsequent step, since the salt concentration is to be, in any case, quite low in the final stage or stages of product treatment.
Other factors being equal, it would be desirable to operate at the highest possible current density in order to maximize the utilization of the ED apparatus. However, as the limiting current density is approached, it is found that water is dissociated (i.e., “split”) into hydrogen ions and hydroxide ions at the interfaces between the (conventional) anion exchange (“AX”) membranes and the diluting streams. The hydrogen ions pass into the diluting streams while the hydroxide ions pass through the AX membranes and into the adjacent concentrate channels that carry a separate solution stream (the “concentrate”, “concentrated”, “concentrating” or “brine” solution or stream, as it is known in the art) that is thereby enriched in ions. Because brackish water may often contain polyvalent metal compounds such as calcium bicarbonate which precipitates at high pH, there is also a tendency for calcium carbonate to precipitate at those surfaces of the AX membranes that are in contact with the concentrating streams. Thus, operation at high limiting current densities results in increased membrane scaling.
This problem has been addressed by a number of techniques, for example, by chemical or ion exchange (IX) softening of the feed water or the concentrating stream; by decarbonation; by adding acid or antiscalant to the feed water or the concentrating stream (with or without decarbonation); by nanofiltration (“NF”); by arranging the dilute and concentrate paths to avoid simultaneously presenting scaling ions and scaling conditions at the same locus; by various cleaning cycles; and by regularly reversing the direction of passage of the electric current and changing the concentrating streams to diluting streams (and the diluting streams to concentrating streams) to drive out scaling species before irreversible deposition can occur, among other approaches. See, e.g., U.S. Pat. Nos. 2,863,813 and 4,381,232.
Of the above techniques, one useful process of general applicability has been the last-mentioned process, namely reversing the electric current, which is referred to in the art as “electrodialysis reversal” (“EDR”).
The theory of limiting current in ED shows that in the case of sodium chloride solution, for example, the cation exchange (“CX”) membranes should reach their limiting current density at values which are about ⅔ that of the anion exchange (“AX”) membranes. Careful measurements have shown that such is indeed the case. However, as the limiting current density of (conventional) CX membranes is approached or exceeded, it is found that water is not split into hydroxide ions and hydrogen ions at the interfaces between such CX membranes and the diluting streams. This difference in behavior relative to the water splitting phenomenon of (conventional) AX and CX membranes at their respective limiting currents has been explained in recent years as being due to catalysis of water splitting by weakly basic amines in the AX membranes. AX membranes which have only quaternary ammonium anion exchange groups (and no weakly basic groups) initially do not significantly split water as their limiting current is approached. Such good behavior continues for only several hours, however, after which period water splitting begins and increases with time. It is found that the AX membranes then contain some weakly basic groups that have resulted from hydrolysis of quaternary ammonium groups. It has been concluded that splitting of water at conventional AX membranes at or near their limiting current densities is an unfortunate phenomenon that, for practical purposes, is unavoidable.
The existence of limiting current in ED also means that in dilute solutions the limiting current densities are relatively very low. For example, at a concentration of salts of about 0.005 kg-eq/m3 (that is about 0.005 g-eq/l or about 300 ppm, a concentration typical of drinking water), the limiting current density is in the range of from about 25 to 50 amperes per m2 (0.0025 to 0.005 amperes per cm2), i.e., the transfer of salts per unit area per unit time is very low (e.g., 50 to 100 grams of salt per hour per square meter). This problem seems first to have been addressed by W. Walters et al. in 1955 (Ind. Eng. Chem. 47 (1955) 61-67) by filling the diluting stream compartments in an ED stack (i.e., an ED apparatus configured as a plurality of flow cells defined by a series of AX and CX membranes) with a mixture of strong base and strong acid ion exchange (IX) granules. The granules or “beads” greatly increase the effective ion exchange surface area in the dilute cells, provide good conductivity between membranes, and provide ion conduction paths across the flow, so that a high degree of demineralization may be achieved during a shorter pass and/or at higher flow velocity. Moreover, a great variety of such beads as well as materials having other conductive, absorptive, exchange or other treatment properties may be employed in the dilute cells. Since 1955, many patents have issued for this technology, among them U.S. Pat. Nos. 3,149,061; 3,291,713; 4,632,745; 5,026,465; 5,066,375; 5,120,416; and 5,203,976. The foregoing patents are hereby incorporated herein by reference.
Two modes of operation using such filled-cell ED (that is, electrodeionization or EDI) have been identified. In the first mode, the exchange (IX) beads serve as extensions of the membrane surface area, thereby greatly increasing the limiting current density. In the second mode, a current density is applied which is very much greater than the limiting current density even with the presence of the IX material. Under these circumstances, the rate of water splitting at membrane-diluting stream interfaces is very high, and the IX granules may then, for example, be predominantly in the strong base and strong acid forms, respectively. The apparatus in this mode is therefore best described as operating as a continuously electrolytically regenerated (mixed bed) ion exchange. An intermediate mode may also be identified in which there is some water splitting but the IX granules are not predominantly in their respective base and acid forms.
Most filled-cell ED (that is, EDI) systems operate in both modes. The modes may occur, e.g., (1) in the same ED cell, operating in the first mode near the entrance to the cell and in the second mode near the exit; (2) in different cells that are placed in flow series between a single pair of electrodes; or, (3) in the cells of separate stacks in flow series (but where each stack possesses its own pair of electrodes). Such different operation may result from the changing conductivity of the fluid as it loses electrolytes traveling along the flow path, or may result from the conscious arrangement of exchange material, cell geometry and operating parameters to achieve this effect.
The economics and the removal capabilities of EDI are quite good, and these systems are used to replace reverse osmosis or conventional, chemically regenerated IX systems, for example, to replace an acid CX column followed by a basic AX column or, at least in part, a mixed bed IX column. A continuously electrically regenerated EDI unit may be filled with less than a cubic foot of exchange resin, replacing an ion exchange bed many times that size and substantially eliminating the costs of periodic resin bed regeneration. However, ion exchange beds and EDI systems are not equivalent in their operational requirements, and each has its own characteristics. When using ion exchange beds (or bottles), the CX and AX granules are periodically chemically regenerated separately from each other, e.g., with aqueous acidic solutions of sulfuric acid or hydrochloric acid, and with aqueous basic solutions of sodium hydroxide, respectively. Precipitates of calcium carbonate, calcium sulfate and magnesium hydroxide are thereby not obtained. When using ion exchange bead columns, the columns of fine granules are also effective filters for colloidal matter, and the trapped material may be rinsed off the granules during their periodic chemical regeneration.
By contrast, in the case of EDI, any calcium or polyvalent metals, bicarbonate and/or sulfate removed from the diluting stream may occur in a higher concentration in the concentrating stream, particularly when the system is operated to achieve high recovery of the diluting stream (which is the usual case when the diluting stream is the intended product, and particularly when it is a high-value product). Elevated concentrations of these species may result in precipitation in the concentrating stream. Furthermore, while it is technically possible to back-wash the IX granules in an EDI apparatus, back washing is generally technically difficult or quite inefficient owing to the confining dimensions of the flow cells in which the exchange beads are packed and to the many small port structures and bead traps generally present in these devices. All of these factors make it problematic to remove any colloidal matter that may accumulate or become trapped in the filled cells.
For many applications, these problems with EDI may be addressed by providing suitable pretreatment processes for the feed fluid itself, for example: (1) regenerable cation exchange for softening followed by regenerable anion exchange absorbents for colloid removal and/or bicarbonate removal; (2) ultrafiltration or microfiltration for colloid removal followed by EDR for softening and partial demineralization; or, (3) ultrafiltration or microfiltration for colloid removal followed by nanofiltration for softening,or by reverse osmosis for softening and partial demineralization.
However, for applications in the food industry, such problems may be more complex. Food syrups may be intrinsically more prone to fouling, and the effective use of ED and its variants for applications such as de-bittering, de-acidification of fluids or precipitation of protein solids may require extremely well controlled process conditions. Protein and other components of alimentary process fluids may be sensitive to the electrochemical conditions prevailing within an EDI stack, and are potentially fouling (for anion resins in particular). Also, the viscosity and temperature requirements of a food process will affect the practically achievable flow and treatment conditions in an EDI or EDI unit. However, the resin and membrane industries have successfully engineered dozens if not hundreds of compatible exchange resins for the food industry where the great prevalence of ion exchange beds and ED still occupy over ninety percent of the treatment market. The range of already-established resins for each product among the strong and weak exchange resins, gel, macroreticular or macroporous bead substrates, high-temperature, specialty resins, adsorbents and other media for fluid treatment thus present a range of ready-made building blocks for applying and improving filled-cell EDI applications.
B. Removal of Poorly Ionized Substances:
ED (including EDR) is used in many food industry plants for alimentary applications, and the range of such applications is increasing, having branched out from predominantly whey processing applications to the processing of fruit juices, modified starch or sugar syrups and separation or refinement of grain slurries, by-product and partially-processed food product streams, as well as fermentation products and specialty chemicals (the latter areas being exemplified, for example, by products such as L-lactic acid from fermentation processes). These ED treatments may allow unexpected efficiencies, such as the return of concentrate salts to an earlier stage to maintain stable culture conditions, the recovery of product “biologicals” that have passed conventional filtration stages, removal of regulated components from a waste stream, or other advantages that increase yield, diminish waste or disposal costs, or otherwise add value to the ED separation process.
However, alimentary process applications present special problems. The treated fluid may be both viscous and complex, involving protein or other components. When one considers, for example, the use of ED to de-ash (“sweeten” or “de-bitter”) cheese whey, generally the natural whey is first concentrated to the range of 20 to 25 percent solids by weight. During ED (or EDR) of such concentrated whey, the current density (that is, the rate of removal of ash per unit area of membrane per unit time) remains relatively high until about fifty to sixty percent of the ash is removed. The remaining ash behaves as if it is poorly ionized, perhaps becoming associated or complexed at lower concentration or shifted pH with protein in the whey. A principal market for de-ashed whey requires ninety percent or higher de-ashing. To de-ash from about forty percent ash down to a ten percent ash level using ED may require much more apparatus contact time than to de-ash from one hundred percent to forty percent ash. This problem may be addressed by the more or less continuous addition of acid to the whey during the later stage de-ashing from forty down to ten percent ash, the acid apparently freeing the ash from the protein. However, the added acid is itself rapidly removed by electrodialysis, introducing a certain inefficiency of operation and requiring elevated doses of acid. An undesirably high quantity of acid is therefore required to complete this process. The problem has also been addressed by removing about the first sixty percent of the whey ash by electrodialysis and applying a different process, such as ion exchange, to remove most of the remaining forty percent. Generally a column of strong acid CX beads followed by a column of weak base AX beads is employed for that purpose. However, considerable quantities of acid and base are then required to regenerate the exchange beads, introducing both direct and environmental costs. The use of specialized ED or other treatment units having cells that are constructed with bipolar membranes which operate to generate acid has also been proposed to address the problem. However, because a relatively large range of concentrations and fairly complex interactions are involved, it remains a lengthy research undertaking to identify a set of processes and operating points that will improve the performance or economics of a given alimentary fluid process.
It would therefore be desirable to mitigate one or more of the above limitations and deficiencies of conventional electrodialysis systems in treating food or similar process fluids.
It would especially be desirable to provide a electrodialysis system architecture that can robustly treat a complex feed stream.